This invention relates in general to demodulation, and more particularly, to a demodulator circuit utilizing a digital circuitry.
Radio frequency identification (RFID) transponders (tags) are usually used in conjunction with an RFID base station, typically in applications such as inventory control, security, access cards, and personal identification. The base station transmits a carrier signal that powers circuitry in the RFID tag when the RFID tag is brought within a read range of the base station. Data communication between the tag and the station is achieved by modulating the amplitude of the carrier signal with a binary data pattern, usually amplitude shift keying. To that end, RFID tags are typically integrated circuits that include, among other components, antenna elements for coupling the radiated field, rectifiers to convert the AC carrier signal to dc power, and demodulators to extract the data pattern from the envelope of the carrier signal.
If fabricated at sufficiently low cost, RFID tags can also be useful in cost-sensitive applications such as product pricing, baggage tracking, parcel tracking, asset identification, authentication of paper money, and animal identification, to mention just a few applications. RFID tags could provide significant advantages over systems conventionally used for such applications, such as bar code identification systems. For example, a basket full of items marked with RFID tags could be read rapidly without having to handle each item, whereas they would have to be handled individually when using a bar code system. Unlike bar codes, RFID tags provide the ability to update information on the tag. However, the RFID technology of today is too expensive for dominant use in such applications.
One factor driving up the cost of conventional RFID tags is the size of the integrated circuit due to the use of analog circuitry. In particular, the circuitry that demodulates the binary pattern that envelops the carrier frequency typically uses analog circuitry such as operational amplifiers and voltage references. Such circuits use precise capacitors and resistors, which are relatively large in size and do not scale with the efficiency of digital circuitry. Further, the design of such circuitry requires very accurate circuit models that are typically only available for mature integrated circuit technologies. Therefore, RFID tags can typically not be manufactured using latest and smallest process technologies that benefit digital circuitry not utilizing analog circuits.
Another concern with demodulators used in convention RFID design techniques is the susceptibility to spikes. FIG. 1 illustrates a typical rectifying and demodulator circuit, as it is known in the prior art. Inductor 104 and capacitor 105 resonate at the carrier frequency. The envelope waveform is isolated on node 102 through the use of band-pass filter 109. The signal is then ac coupled through a capacitor to node 108, creating a short pulse in the negative or positive direction at the input of comparator 107. A high pulse indicates a xe2x80x9chigh statexe2x80x9d in the envelope while a low pulse indicates a xe2x80x9clow statexe2x80x9d in the envelope. Differential amplifier 107 compares these pulses with a voltage reference 101, and will generate a xe2x80x9chighxe2x80x9d state or a xe2x80x9clow state on output 103. In essence, this circuit isolates the envelope data signal by sensing the rise/fall time of the envelope signal. Since noise spikes tend to have fast rise and fall times, noise spikes tend to create erroneous output states. Further, since output 103 changes state only on the next rise/fall time of the envelop data of the carrier signal, output 103 will remain in an incorrect state until the next data transition is detected.
According to principles of the present invention, a demodulator converts a voltage input to an output voltage. The demodulator has a voltage-controlled oscillator (VCO), a counter, a holding apparatus, and a digital compare apparatus. The VCO generates a signal having a frequency proportional to the analog input voltage. The counter counts each cycle of the signal generated by the VCO and outputs a count signal representative of the cycle count. The holding apparatus holds the count signal and generates a held count signal. The digital compare apparatus compares the count signal and the held count signal and generates the digital output.
According to principles of the present invention, the VCO has an input terminal and output terminal. The counter has an input terminal, an output terminal, and a reset terminal. The holding apparatus has an input terminal, an output terminal, and a reset terminal. The digital compare apparatus has first and second input terminals and a reset terminal. The voltage input is coupled to the input terminal of the VCO, the output terminal of which is coupled to the input terminal of the counter. The output terminal of the counter is coupled to the input terminal of the holding apparatus. The output terminal of the counter and the output terminal of the holding apparatus are coupled to first input terminal and second input terminal of the digital compare apparatus. The reset terminal of the counter, holding apparatus, and digital compare apparatus are coupled to the reset clock.
According to further principles of the present invention, the VCO has includes an n-channel MOSFET device, a storage capacitor, a p-channel MOSFET, and a Schmidt Trigger. The n-channel MOSFET device has source, drain, and gate terminals. The storage capacitor has first and second terminals. The p-channel MOSFET has drain, source, and gate. The Schmidt Trigger has input, output, and reset terminals. The gate of the n-channel MOSFET is coupled to the input voltage, the source is coupled to ground, and the drain is coupled to the first terminal of the storage capacitor. The second terminal of the storage capacitor is coupled to the power supply. The drain of the pchannel MOSFET is coupled to the first terminal of the storage capacitor and the input terminal of the Schmidt trigger. The source of the p-channel device is coupled to the power supply. The gate of the p-channel MOSFET is coupled to the output of the Schmidt trigger. The output of the Schmidt trigger provides the output voltage.